code: def load(
      self, instruments=None, start_time=None, end_time=None
  ) -> pd.DataFrame:
      df = super(Avg15minLoader, self).load(
          instruments, start_time, end_time
      )
      if self.is_group:
          # feature_day(day freq) and feature_15min(1min freq, Average every 15 minutes) renamed feature
          df.columns = df.columns.map(
              lambda x: ("feature", x[1])
              if x[0].startswith("feature")
              else x
          )
      return df
  
file_path: qlib/examples/benchmarks/LightGBM/multi_freq_handler.py
imports: import pandas
  from qlib.data.dataset.loader import QlibDataLoader
  from qlib.contrib.data.handler import DataHandlerLP, _DEFAULT_LEARN_PROCESSORS, check_transform_proc
label_desc: Use image node_id 1 for calling the Avg15minLoader obj's underlying member method code with example usage: obj.load(instruments, start_time, end_time) and returns: df